The invention concerns a process for drying a material web, specifically a paper or cardboard web, where the material web along with a preferably porous backing belt runs alternately across contact zones on heatable rolls contacted by the material web and across guide rolls which are contacted by the backing belt and where in the sections of the web travel contained between two contact zones drying air is supplied to the free web surface. Additionally, the invention concerns a device for the application of this process.
The invention is based on a prior process and a prior drying apparatus serving the application of the prior process where the material web runs together with the backing belt alternately across upper and lower rolls, for instance drying cylinders. The material web is supported along the entire way through the drying apparatus by the backing belt, so that the material web need not run freely from one roll to another, except perhaps at the exit from the drying apparatus.
Such drying apparatuses or "drying groups" have proved themselves specifically in paper machines. They are used primarily in the initial area of the drying process, where the paper web is still rather moist and thus has only a low inherent strength. Prior drying apparatuses of this type have been described in the following documents: U.S. Pat. No. 3,503,139; U.S. Pat. No. 4,064,637; U.S. Pat. No. 4,625,434; WO 83/00514; and German Patent disclosure No. 35 20 070 (equivalent to U.S. Pat. No. 4,625,430).
In all of these prior drying apparatuses, supplying the heat to the paper web to be dried takes place primarily in the contact zones, that is, by direct heat transfer from the outer cylinder surface of the heatable rolls (preferably of the steam-heated drying cylinders) to the paper web. Additionally, drying air is fed in these prior drying apparatuses (in a more or less intensive way) to the exposed web surface. This always occurs in the sections of the web travel which are contained between two contact zones. The objective of this arrangement is to improve or hasten the drying process. According to WO 83/00514, for instance, hot air blowing boxes are provided for supplying drying air in the area of the so-called suction guide rolls. These latter serve to hold the paper web on the backing belt. The drying air supplied through the hot air blowing boxes can absorb part of the steam released from the paper web. Details concerning removal of the drying air laden with steam are not taught in WO 83/00514.
Under previously customary conditions, i.e., specifically at the previously customary maximum paper machine speeds in the order of about 1,000 to 1,200 m/min, the prior drying methods and drying apparatuses have yielded more or less satisfactory results. Said maximum operating speeds apply to the production of graphic papers in particular. The production of tissue papers already proceeds today at operating speeds up to about 1,900 m/min, but using different drying methods.
Recently, maximum operating speeds considerably higher than heretofore are also being sought in the production of graphic papers. Attempts are being made to raise the maximum operating speed to 1,500 m/min and even higher. To accomplish this goal, the prior drying methods and drying apparatuses need to be improved in many respects, as follows.
1. The specific drying capacity must be increased. The objective in this is to have the length of the drying apparatus not significantly exceed the previously customary dimensions, seeking rather a shortening of the overall length.
2. The efficiency or economy of the drying process must be improved so as to keep the energy consumption within reasonable limits.
3. The so-called run efficiency (runability) of the drying apparatus should be maximally high, i.e., downtimes caused by any paper web breaks should be kept as low as possible.
4. In case a web break still occurs now and then, provisions need to be made that the accruing scrap can be removed with as little risk as possible.
5. In the start-up or restart of the paper machine, the threading of the paper web (which, as is generally known, must take place at the full operating speed) must take place automatically at high reliability.
6. The already known cross profile control in drying (by subdivision of the drying air influx in individually controllable partial flows) should continue to be possible and made still more effective.
The problem underlying the invention is to improve the initially described drying process and the pertaining drying apparatus to the effect that the operating speed can be raised considerably as compared to before and that, at the same time, the above-stated requirements will be met extensively.
Austrian patent No. 308,524 describes a drying process of a different category. There, the paper web passes without a backing belt over a single drying cylinder. Hence, there is only a single contact zone in which the paper web contacts the drying cylinder with its one side. Within this contact zone, drying air is supplied to the other side of the paper web (in the area of pressure chambers). Located between two pressure chambers each is a suction opening through which steam-laden drying air can be removed.